1. Field of the Invention
The present invention relates to the injection-molding of pasty organopolysiloxane compositions, which compositions can be readily transferred or circulated by means of pumps and which are conveniently thermoset to form elastomers. The subject compositions are constituted by intimately admixing diorganopolysiloxane oils, finely divided silicas, structuring inhibitors and a cross-linking agent, the latter being 2,4-dichlorobenzoyl peroxide.
2. Description of the Invention
The prior art relating to the technology of the manufacture of silicone elastomers which are vulcanized under the action of heat and derived from rubbery and hence non-pasty compositions (formed by mixing diorganopolysiloxane gums, inorganic fillers, structuring inhibitors and organic peroxide cross-linking agents) reflects that the choice of the cross-linking agents is closely dependent upon the conditions of vulcanization of the compositions and on the forming apparatus adapted to these conditions.
Thus, 2,4-dichlorobenzoyl peroxide is widely employed for hardening extruded compositions at high temperatures, preferably at about 200.degree.-400.degree. C., in a hot oven, in the ambient atmosphere [U.S. Pat. No. 2,723,960]. On the other hand, it is employed only with discretion in injection molding techniques because, on account of its relatively low decomposition temperature and its rapid rate of decomposition, it is likely to cause the premature hardening of the injection-molded compositions before the molds have been completely filled; this is referred to as scorching (Lynch, Handbook of Silicon Rubber Fabrication, pages 35, 36 and 37). It is thus preferred to use dicumyl peroxide and 2,5-bis-(t-butylperoxy)-2,5-dimethylhexane in connection with industrial injection-molding machines (abovementioned text of W. Lynch, pages 66, 67 and 71).
The use of 2,4-dichlorobenzoyl peroxide (together with the use of other peroxides) is described in U.S. Pat. No. 3,791,998 for hardening, not rubbery compositions, but pasty compositions formed by mixing diorganopolysiloxane oils having a viscosity on the order of 100,000 to 750,000 mPa.s at 25.degree. C., inorganic fillers and structuring inhibitors. These compositions are deposited or coated onto fabrics and then hardened, and the materials obtained are used for electrical insulation. The two examples in the noted patent illustrate the use of 2,4-dichlorobenzyl peroxide:
(i) in Example 1, the composition used is hardened by heating for 10 minutes at 160.degree. C., in the atmosphere; and PA0 (2i) in Example 2, the same composition is hardened by heating for 3 minutes at a temperature of not more than 120.degree. C., in a mold; it is subjected to compression molding. It will be appreciated that no reference is made in this patent to the injection-molding technique. PA0 (A.sub.1) 100 parts of a diorganopolysiloxane oil having a viscosity of 500 to 300,000 mPa.s at 25.degree. C., consisting essentially of recurring units of the formula R.sub.2 SiO and blocked at each end of the chain by units of the formula R.sub.2 R'SiO.sub.0.5, in which formulae the symbols R, which are identical or different, represent hydrocarbon radicals which are unsubstituted or substituted by halogen atoms or cyano groups and which have from 1 to 8 carbon atoms, and the symbol R' is defined exactly as were the symbols R and also represents a hydroxyl radical, an alkoxy radical having from 1 to 4 carbon atoms or a .beta.-methoxyethoxy radical. PA0 (B.sub.1) 10 to 75 parts of finely divided, reinforcing silicas having a specific surface area of at least 50 m.sup.2 /g. These silicas can be untreated or treated; when the silicas are treated, this is advantageously carried out to a degree of at least 5% with commercially available organosilicon compounds providing units selected from among those of the formulae (CH.sub.3).sub.2 SiO, (CH.sub.3)(CH.sub.2 .dbd.CH)SiO, (CH.sub.3).sub.3 SiO.sub.0.5 and (CH.sub.3).sub.2 (CH.sub.2 .dbd.CH)SiO.sub.0.5, PA0 (C.sub.1) 1 to 20 parts of structuring inhibitors, and PA0 (D.sub.1) 0.1 to 4 parts of 2,4-dichlorobenzoyl peroxide, and said injection-molding thereof being carried out at a temperature ranging from 125.degree. to 225.degree. C. The subject process is quite advantageous in that it permits a hardening time of less than 15 seconds and a molding cycle of less than 25 seconds. PA0 (i) alkyl and halogenoalkyl radicals having from 1 to 3 carbon atoms, such as methyl, ethyl, propyl, isopropyl and 3,3,3-trifluoropropyl radicals; PA0 (ii) alkenyl radicals having from 2 to 4 carbon atoms, such as vinyl, allyl and but-2-enyl radicals, PA0 (iii) cycloalkyl and halogenocycloalkyl radicals having from 5 to 6 nuclear carbon atoms, such as cyclopentyl, cyclohexyl, methylcyclohexyl and chlorocyclohexyl radicals, PA0 (iv) mononuclear aryl and halogenoaryl radicals having from 6 to 8 carbon atoms, such as phenyl, tolyl, xylyl, chlorophenyl, dichlorophenyl and trichlorophenyl radicals; and PA0 (v) cyanoalkyl radicals of which the alkyl radicals have from 2 to 3 carbon atoms, such as .beta.-cyanoethyl and .gamma.-cyanopropyl radicals. PA0 (CH.sub.3).sub.2 SiO, (CH.sub.3)(CH.sub.2 .dbd.CH)SiO, CH.sub.3 (C.sub.6 H.sub.5)SiO, (C.sub.6 H.sub.5).sub.2 SiO, PA0 C.sub.6 H.sub.5 (CH.sub.2 .dbd.CH)SiO, CF.sub.3 CH.sub.2 CH.sub.2 (CH.sub.3)SiO, PA0 NC--CH.sub.2 CH.sub.2 (CH.sub.3)SiO, NC(CH.sub.2).sub.3 CH.sub.3 SiO PA0 (CH.sub.3).sub.3 SiO.sub.0.5, CH.sub.2 .dbd.CH(CH.sub.3).sub.2 SiO.sub.0.5, (CH.sub.3).sub.2 C.sub.6 H.sub.5 SiO.sub.0.5, PA0 CH.sub.3 (C.sub.6 H.sub.5)(CH.sub.2 .dbd.CH)SiO.sub.0.5, HO(CH.sub.3).sub.2 SiO.sub.0.5, HO(CH.sub.3)(CH.sub.2 .dbd.CH)SiO.sub.0.5, PA0 CH.sub.3 O(CH.sub.3).sub.2 SiO.sub.0.5, CH.sub.3 CH.sub.2 O(CH.sub.3).sub.2 SiO.sub.0.5, CH.sub.3 OCH.sub.2 CH.sub.2 O(CH.sub.3).sub.2 SiO.sub.0.5 PA0 (1) (if R'=R), low molecular weight diorganopolysiloxanes of the formula R.sub.3 SiO(R.sub.2 SiO).sub.x SiR.sub.3, x having a value sufficient to provide a viscosity ranging from 0.5 to 100 mPa.s at 25.degree. C., PA0 (2) (if R'=OH), water and/or oil of the formula HOR.sub.2 SiO(SiR.sub.2 O).sub.y SiR.sub.2 OH, y having a value sufficient to provide a viscosity ranging from 5 to 200 mPa.s at 25.degree. C., and PA0 (3) (if R'=alkoxy or CH.sub.3 OCH.sub.2 CH.sub.2 O), the corresponding alcohol R'H and/or a low molecular weight oil of the formula R'R.sub.2 SiO(R.sub.2 SiO).sub.z SiR.sub.2 R', z having a value sufficient to provide a viscosity ranging from 0.5 to 120 mPa.s at 25.degree. C. PA0 (i) diorganopolysiloxane oils having low viscosities on the order of 5 to 500 mPa.s at 25.degree. C., and blocked at each end of their chain by a hydroxyl radical and/or an alkoxy radical having from 1 to 3 carbon atoms. The organic radicals bonded to the silicon atoms in these oils are preferably methyl, ethyl, vinyl, phenyl or 3,3,3-trifluoropropyl radicals. PA0 (ii) diphenylsilanediol and the silanes of the formulae: ##STR1## PA0 [1] an injection cylinder housing a screw or a piston, the front of which is equipped with an injection nozzle; PA0 [2] a pressure mechanism transmitting, to the piston or to the screw, the force necessary for injecting the products; PA0 [3] a mold with two dies, opening at the rear; and PA0 [4] a mold-locking device ensuring the closing and opening of the mold and the ejection of the moldings.
However, reference is indeed made thereto in another patent, namely, U.S. Pat. No. 4,173,560, which describes compositions which are fairly similar to the above compositions and are formed by mixing diorganopolysiloxane oils having, per mol, about 2 vinyl radicals bonded to the silicon atoms, and preferably having a viscosity of 200 to 100,000 mPa.s at 25.degree. C., silicas treated with vinylic amidoorganopolysiloxanes, and a cross-linking system consisting of organic peroxides or combinations of organohydrogenpolysiloxanes and platinum derivatives; 2,4-dichlorobenzoyl peroxide is included among the peroxides, but the preferred peroxides are di-t-butyl peroxide, t-butyl perbenzoate and 2,5-bis-(t-butylperoxy)-2,5-di-methylhexane.
Said U.S. Pat. No. 4,173,560 also teaches that the compositions which are destined for molding in a "liquid" injection-molding process have a well-defined viscosity which must not exceed 160 Pa.s, this value probably being determined at 25.degree. C. (column 6, lines 20 to 26 of U.S. Pat. No. 4,173,560). Compositions of this type, therefore, have a relatively low viscosity.
And for their manufacture, these compositions, and those which can be molded by another technique, require the incorporation of finely divided silicas treated in accordance with a process featuring organosilicon compounds (the abovementioned vinylic amidoorganopolysiloxanes) which are not readily available on the silicone market, and solvents. They are hardened in accordance with the conditions of temperature and time normally used by those skilled in the art of silicone elastomers (column 6, lines 5 and 6 of U.S. Pat. No. 4,173,560).
Only Example 3 of the said patent illustrates the use of a peroxide, namely, 2,5-bis-(t-butylperoxy)-2,5-dimethylhexane. The compositions containing this peroxide are heated in a mold for 15 minutes at 175.degree. C.; they are subjected to compression molding; injection molding of these compositions is not illustrated.
Injection molding makes it possible to manufacture high-quality silicone elastomer moldings at rapid rates. In the case of injection-molding rubbery organopolysiloxane compositions, same demands powerful machines which are frequently heavy and expensive (pages 64 to 83 of the aforesaid Lynch text, Handbook of Silicon Rubber Fabrication). However, in the case of injection molding the so-called fluid compositions (those of U.S. Pat. No. 4,173,560), such molding can be carried out on inexpensive, light machines, for example of the type used for injection-molding low-viscosity and medium-viscosity plastics in the molten state. It would be advantageous, from an industrial point of view, to adapt these light machines to injection-moldable compositions which are less fluid than the above compositions and are manufactured from finely divided silicas which are untreated or have been treated with organosilicon compounds which are very readily available on the commercial chemical products market, and which contain 2,4-dichlorobenzoyl peroxide as the cross-linking agent. As above indicated, this peroxide has a very rapid decomposition rate, which would make it possible, for example, to increase the number of moldings or mold cycles per unit time, compared with 2,5-bis-(t-butyl-peroxy)-2,5-dimethylhexane.